Discharge tube



Feb. '11, 1969 I HIDEO MIZUNO ET AL DISCHARGE TUBE File d April 17, 1967 FREQUENCY (KC) United States Patent 3,427,491 DISCHARGE TE Hideo Mizuno, Talratsulri-shi, Hidezo Alrutsu, Suita-shi,

Eijiro Moriguchi, Kyoto, Katsuyuki Yamashita, Takatsuki-shi, Shigeru Karniya, Hirakata shi, Koshi Iwata, Takatsuki-shi, Yoshio Tawara, Kadorna-shi, and Atsushi Iga, Hirakata-shi, Japan, assignors to Matsushita lElectronics Corporation, Osaka, Japan, a corporation of Japan Filed Apr. 17, 1967, Ser. No. 631,221 Claims priority, application .iapan, Apr. 20, 1966,

il/25,630 Us. or. 313-346 1 Claim rm. Cl. Htllj 1/14, 19/06; Hark 1/04 ABSTRACT OF THE DISCLOSURE The present invention relates to an improvement in discharge lamps, and more particularly to an improvement of low presure mercury discharge tubes such as fluorescent lamps.

Oxide cathodes of the prior art utilized as electrodes in low pressure mercury vapor type discharge tubes such as fluorescent lamps were formed by coating double coils or so-called triple coils of tungsten having a fine filament wound around said double coil of tungsten with a compound carbonate consisting of barium carbonate, strontium carbonate and calcium carbonate, enclosing the resulting coated structures in a discharge tube, thereafter subjecting these carbonates to thermal decomposition during the process of exhausting gas from said tube, thereby producing oxides of barium, strontium and calcium. The oxide layers thus produced were of high resistivity and poor thermal conductivity. Accordingly, a discharge tube incorporating cathodes thus obtained led, when lighted, to the development of cathode spots locally having a high temperature and such cathode spots constituted the centers of thermionic emission. When such discharge tube of the prior art was lighted, this temperature of the cathode spots did not make any substantial change, due to their thermal inertia, either during the re-ignition or extinction of the arc of the cathodes in all AC cycles, but rather the cathode spots continued to have a high temperature. Therefore, at the time of re-ignition and extinction of the are at the cathodes of the prior art, the thermionic current I naturally became larger than the discharge current 1;, causing a negative potential to appear in front of the cathodes, and this led to the development of cathode oscillation such as re-ignition oscillation and extinction oscillation, resulting in the occurrence of a severe radio-interfering noise.

In order to avoid the generation of such interfering noise in fluorescent lamps, various attempts have been made to improve the thermal conductivity of the oxide layers of the electrodes used in such lamps. For example, Japanese patent publication No. 1,581/ 1964 proposes lowering the temperature of the cathode spots by restricting the thickness of the oxide layers to 30p or less to sub stantially enhance the thermal conductivity of cathodes and to thereby enlarge the cathode spots. However, this prior method is not desirable because it inevitably reduces the absolute volume of the electron-emitting oxides, and this in turn, reduces the life of the discharge tube. Another attempt has been reported in Japanese patent publications N0. 8,391/1965 which proposes enlargement of the distances between the pitches of coils to prevent an excess elevation of the temperature in coils due to mutual radiation occurring within the coils to thereby enlarge the cathode spots and lower the temperature thereof. Nevertheless, lamps having such arrangements produce radiointerfering noise in the order of 35 db when lighted and, therefore, they still are unsatisfactory for use as fluorescent lamps sufiiciently low in radio-interfering noise.

It is the object of the present invention to improve discharge lamps and more particularly to provide low pressure mercury discharge tubes such as fluorescent lamps producing very little radio-interference in the broadcasting frequency band of from 535 kc. to 1605 kc.

The inventors have discovered that a fluorescent lamp for practical use, which is substantially noiseless, namely, producing a radio-interfering noise as low as 15 db or less and which is free from the development of blackening of the tube is obtained by the use of a cathode emitter consisting principally of oxides of barium, stronium and calcium, said oxides containing iron-cobalt borides having high melting points and having a markedly superior thermal conductivity as compared to ordinary ionic crystals and having been stabilized by substituting a part of said iron with one or more of the substances selected from the group consisting of titanium, zirconium, hafnium, vanadium, nobium, tantalum, chromium, molybdenum, tungsten, aluminium and silicon, and further, containing powder of a reducing metal selected from the group consisting of zirconium, hafnium, niobium and tantalum.

Powders of said reducing metals, namely, zirconium, hafnium, niobium and tantalum satisfactorily prevent the metal borides from being oxidized during the thermal decomposition of the carbonates, which is conducted during the process of exhausting gas from the discharge tubes.

According to the result of the experiment conducted by the inventors, the discharge tube embodying the present invention with its oxide cathodes being made of a material consisting of oxides of barium, stronium and calcium and containing 1% by weight of metal borides corresponding to the composition formula of and 3% by weight of zirconium, relative to the total weight of said oxides, respectively, is of a remarkably superior noise minimizing effect to conventional discharge tubes wherein the cathodes are made of such oxides. By subjecting a fluorescent lamp of the present invention to a test which was accomplished by connecting a commonly known parallel condenser having 0.006 ,uf., an epochmaking ability represented by a noise intensity as low as 15 db or less throughout the broadcasting frequency band between 535 kc. and 1605 k.c. was obtained.

The drawing shows a comparison 'between the magnitudes of noise imparted to a radio-receiver by a fluoroescent lamp embodying the persent invention and by a conventional fluorescent lamp.

In the drawing, curve 1 represents the magnitude of the noise, caused by a fluorscent lamp in which a conventional oxide is used.

The causes for such a remarkable decrease in the cathode oscillation as is indicated by curve 2, when oxide cathodes contain metal borides, have not been elucidated as yet, but in view of the finding in said experiment that the cathode spots were larger than those of conventional oxide cathodes and the finding that the temperature of the cathode spots of the former was lower than that of the latter, it is conjectured that one of the causes may be represented by the fact that metal borides have a much higher thermal conductivity as compared with oxides of barium, strontium and calcium.

Viewing from the superior effect of noise reduction produced by the metal borides, it is conjectured that there may be present a certain relationship between the metal borides and the electron-emitting oxides.

The metal borides used in the present invention point to such poly-element metal borides as are represented by the composition formula of wherein Me represents one or more of the metals selected from the group consisting of titanium, zirconium, hafnium, canadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminium and silicon. A mixture of the aforesaid metal borides and carbonates of barium, strontium and calcium is applied to the surfaces of the coils of electrodes. The coated electrodes are heated to the order of 1200 C. during the process of exhausting the gas from the fluorescent lamp, and as a result the aforesaid carbonates are decomposed by the heat to form the so-called oxide electrodes.

It is, therefore, necessary that the added metal borides have a melting point of 1300 C. or higher and that they do not become oxidized when heated to a temperature of the order of 1200 C. in a carbon dioxide gas atmosphere.

As for iron boride there are two different composition ratios, namely, Fe B and FeB. Cobalt boride includes two different composition ratios, namely, C 8 and CoB. Their melting points are 1389 C., 1550 C., 1265 C. and 1350 C., respectively. Fe B and Co B have melting points which are slightly lower than those of FeB and CoB. However, a metal boride containing boron in a larger proportion as in FeB and CoB markedly reduces the life of the cathode emitter. For example, a fluorescent lamp wherein the cathodes are made of an emitter containing by weight of FeB has a life duration of about 2000 hours. However, a fluorescent lamp whose cathodes are made of an emitter containing 5% by weight of Fe B has a life duration of the order of 5000 hours. A lamp wherein the cathodes are made of an emitter containing 1% by weight of Fe B has a life duration of 8000 hours or more. It is, therefore, desirable to use iron-cobalt borides whose composition formula is (Fe-Co) B. In case the proportion of the volume of boron is smaller than that indicated by the chemical composition, however, this leads to the development of free iron or cobalt, which as has been already described, resulting in the metal boride becoming susceptible to oxidation. For this reason, it is desirable to use a volume of boron somewhat larger in proportion, i.e. z=1 to 1.15 in the aforesaid composition formula, than is indicated by the chemical composition. It is to be noted, however, that in case z 1.15, the duration of life of the lamp is affected, and therefore, the use of boron in a proportion greater than this limit should be avoided.

In case the metal boride consists of a simple substance of Fe B, this substance becomes oxidized to some extent during the decomposition of the electrodes, resulting in the end-band type blackening after about 1000 hours of lighting. This blackening is caused by the mercury oxide particles formed by the coupling of the residual impure gases, especially, oxygen with mercury, depositing on the inner peripheral face of the tube in the area about 3 cm. in front of the electrodes.

While this end-band type blackening has nothing to do with the duration of life of the fluorescent lamp, it seriously affects the value of the lamp as a commodity. Co B is superior to FeB in resistivity to oxidation at high temperature, the occurrence of blackening of this nature can be substantially arrested in the case where (30 B is 4 added to the cathode emitter. In view of the fact, however, that C0 13 has a low melting point, 1265 C., the use of COZB as a simple substance will develop yellowish brown blackening in the terminal portions of the tube behind the electrodes after about 1000 hours of lighting. In contrast to this, when a substance consisting of (in which 0.15x0.6) wherein 15 to 60% of iron of Fe B has been substituted with cobalt is used, the development of the end-band blackening and the development of blackening in the terminal portions of the tube are both greatly improved as compared with the use of the simple substance of R2 8 or Co B. In the foregoing composition, when x5015, the lamp is susceptible to end-band blackening as in the tube where Fe B as the simple substance is used. In case x 0.6, the lamp is susceptible to blackening in the terminal portions of the tube as is so with the tube using the simple substance of Co B.

The metal borides which are used in the present invention are comprised of poly-element borides which are composed principally of the iron-cobalt borides indicated by the composition formula of wherein a part of the iron has been substituted by one or more of the substances selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminium and silicon. In the composition formula, y has a value ranging from 0.01 to 0.3.

Borides of titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminium and silicon have very high melting points. The melting point of Mo B is 2000 C., that of Ti B is 2790 C., that of Zr B is 3040 C. and that of W 13 is 2770 C. Thus, they have melting points which lie in the level of 2000 C. or higher. In view of the fact that these substances have a high resistivity to oxidation, and remain chemically stable at high temperature, as compared with the simple ironcobalt borides, the discharge lamp employing an emitter containing the aforesaid composition in Which the aforesaid borides are added has the superior features that it develops very little amount of blackening during the course of lighting and also that its effect of minimizing radiointerfering noise is hardly reduced. The metals having melting points such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and aluminium, and silicon can be substituted in the range of amount, i.e. y=0.0l to 0.3, and preferably, in the range of from 0.03 to 0.1. In case y, which indicates the amount to be substituted by these metals having high melting points, takes a value less than 0.01, the effect of improving the stability of the emitter at high temperature is reduced, while on the other hand, when it exceeds 0.3, it is rather difficult to make solidsolution of borides.

In the oxide cathodes containing the aforesaid metal borides, the effect of minimizing to noise interference is noted when the cathodes contain 0.05% or more by weight of such metal borides. In order to avoid variance in the quality of goods during the process of this manufacture and to obtain goods of uniform and stable quality, it is preferred to add the borides in a proportion of the order of from 0.1 to 2%. While the effect of minimizing the noise interference can be equally obtained from the addition of metal borides of more than 2% by weight, the addition of metal borides in a proportion in excess of 10% by weight means a reduction in the proportion of oxides of barium, strontium and calcium and results in a reduced duration of life of the lamp, and therefore, the addition of metal borides in such excess proportion is not desirable.

As has been previously described, the addition of an additive consisting of powder of one or more of the reducing metals having high melting points selected from the group consisting of zirconium, hafnium, niobium and tantalum which is to be contained in a proportion of the order of from 1 to 8% by weight in the oxides of the cathodes of the present invention is mandatory for the prevention of blackening of the lamp during lighting and also for the purpose of obtaining a prolonged duration of life of the lamp. The use of these additives in an amount less than 1% by weight leads to a failure in obtaining the desired effect, while the addition of an amount in excess of 8% by weight leads to a reduction in the ability of the cathodes to emit electrons therefrom, and therefore, the employment of these two extremes is not desirable.

EXAMPLE 1 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.4Fe'O.4Co-0.05Ti-0. 05Al) B l Zirconium 3 EXAMPLE 2 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.5Fe- 0.4Co-0.03Zr-0.02Hf) l3 1 Zirconium 3 EXAMPLE 3 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 Zirconium 2 Hafnium 1 EXAMPLE 4 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.7Fe-0.2Co-O.05W) B 1 Zirconium 2 Hafnium 1 EXAMPLE 5 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.7Fe-0.2Co-0.-03V-0. O2Nb) B 1 Zirconium 2 Niobium 1 EXAMPLE 6 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.7Fe-0.2-Co-O.03Ta-0.02Si) B 1 Zirconium 2 Niobium 1 EXAMPLE 7 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.5Co-0.4Fe-0.05Cr) B l Zirconium 2 Tantalum 3 6 EXAMPLE 8 Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.5-Co-0.4Fe-0.03Cr-0.02Mo) B 1 Zirconium 2 Tantalum 3 The mixtures shown in the above examples, suspended in butyl acetate solutions of nitrocellulose, were applied to the double coils and the triple coils of tungsten. The resulting coated coils were then subjected to thermal decomposition during the gas-exhausting process performed on the discharge tubes. Low pressure mercury vapor type discharge tubes having oxide cathodes consisting of these processed coils were thus fabricated. Ternary carbonates having the previously described composition ratios were used as the barium carbonate, strontium carbonate and calcium carbonate as are enumerated above.

The fluorescent lamps fabricated in the manner described above were subjected to test by connecting them to commonly utilized parallel condensers of 0.0 0 6 ,uf. All of these lamps showed that the interfering noise in the frequency band of from 535 kc. to 1605 kc. was 15 db or less as is indicated by curve 2 in the drawing, and also that the duration of life was similar to that of ordinary discharge tubes, Thus, the lamps manufactured according to the present invention could be termed as being substantially perfectly noiseless fluorescent lamps.

We claim:

1. A discharge tube satisfactorily low in radio-interfering noise and equipped with cathodes coated with a cathode emitter consisting of oxides consisting principally of oxides of barium, strontium and calcium, said oxides containing, in a proportion of from 0.05 to 10% by weight relative to said oxides, a substance having the composition formula of and consisting principally of iron-cobalt borides of which a part of said iron having been substituted with one or more metals Me selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminum and silicon, the mivture of said oxide and said substance containing, in a proportion of from 1 to 8% by weight relative to said oxides, one or more reducing metals having high melting points and selected from the group consisting of zirconium, hafnium, molybdenum and tantalum.

References Cited UNITED STATES PATENTS 2,473,358 6/1949 Bright 313-345 X 2,724,070 11/1955 Heine et a1. 313-337 2,820,920 1/1958 Penon 313345 X 2,849,637 8/1958 Weiss 313-345 X 3,312,856 4/1967 Lafferty et a1. 313-34 6 JOHN W. H-UCKERT, Primary Examiner.

I. R. SHEWMAKER, Assistant Examiner.

US. 01. XiR. 

